Author Topic: MS 462 Carb issue  (Read 148 times)

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Offline joe_indi

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MS 462 Carb issue
« on: October 09, 2021, 04:21:20 am »
Though 'God' (Stihl) has banished me from Eden  ;) some independent saw repair individuals contact me if they have an undetectable issue
This time it was a four month old 462. Idles and runs at around 5000 rpm but rev up and the engine cuts off as if starved for fuel.
He replaced the pickup body, tried another coil, the carb from another non running 462. Same issue
So he called me and I told him to remove the clutch and worm, start up the saw, drop som oil onto the oil seal and try revving. If there is some smoke before it dies and the oil disappears, replace the seal.
That didnt work
He had pressure tested the carb, dismantled it, reassembled it, repeated the pressure test, no sign of leaks
He tried it with another carb, this time from a saw that had come in for a chain (that was a troublefree saw)
This time it ran fine!
So, we finally established the carb was the culprit.
I told him to repeat the pressure test but this time instead of the 0.2 or 0.3 bar try for 1 bar. No pressure drop
But, after 0.5 bar a slight drop commenced and at 0.8 the drop was rapid and he called to say it wouldnt go any further. But when the pressure dropped below 0.5 bar there was no more pressure loss.
I told him to pump the bulb rapidly to hit 1 bar.
He called again. He hit 1 bar and after that the needle swung to zero and the pressure was no longer holding at any point.
He opened up the pump side cover and sent a picture of the diaphragm
I had  developed a tear!
So I asked him to have a look at the carb body. The edge of the body where the diaphragm goes back and forth with the impulse was extremely sharp.
If this could happen once it could happen again. So he had a look at the other saw from which he had used the first 'test carb'
Edge was sharp and now that he examined with care he could detect a not yet torn area where light came in. Next he opened up the carb of the saw from which he had used the second (successful ) 'test carb'
It had the makings of a possible diaphragm failure in the near future  :D
I advised him to use a suitable knife to shave off the sharp edge on that carb.
I hope that fixes the issue.
So just in case you receive any 462 on your work table have a look at the carb's pump side. And if it looks like the one in the picture, pick up your handy knife and get to work  ;D

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Offline Cut4fun .

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Re: MS 462 Carb issue
« Reply #1 on: October 09, 2021, 07:15:48 am »
I got a new MS290 like that once. Was a slit cut on check valve side in circle part way. Crazy on a like new saw just like that one.

Great find. Those are the fun trouble shooting ones to figure out in the end. 
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